The present invention relates to porous tubes and other hollow profile structures, and to methods of making such structures. The invention also relates to a method of making shaped microporous articles particularly useful in making the above structures.
Porous, particularly micro-porous, tubes are increasingly being used in many diverse applications. One broad application is gas disperser systems, such as fish pond aeration, waste-water treatment, and the like; other applications include liquid distribution systems, such as drip irrigation or chemical processing systems. Many techniques are known for producing the porous tubes, e.g., by including a leachable additive or a blowing agent within a plastics matrix. However, the porous tubes are quite expensive to produce according to the conventional techniques, and moreover they have low mechanical strength. These drawbacks limit their potential applications.
Among the objects of the present invention are to provide a new method of making porous tubes and hollow profile structures, and to provide new porous tube structures, which method and structures have advantages in the above respects.
Further, a known method of producing shaped microporous articles involves a leaching process in which the two main components are a thermoplastic material and a leachable polymer material. These materials are basically incompatible with each other, and are usually available in different physical forms, the thermoplastic matrix material being granular, and the leachable polymeric material being powdered. In the known process, these two materials are intimately mixed under heat and pressure, such as by kneading or milling, until a uniform blend is obtained. The blend is cooled, shaped by a thermoplastic (heating) process, such as extrusion, calandering or moulding; and then the leachable polymeric material is removed, thereby resulting in the shaped microporous article.
The above known technique has several drawbacks: Thus, the mixer must compound both of the polymeric components during the step in which they are mixed under heat and pressure, this being done in special kneading or milling machines which are very expensive. In addition, because the mixer must accommodate both components, the apparatus required must be relatively large. Further, in the conventional technique the thermoplastic matrix material is subjected to two heating operations, one during the above mixing step and the other during the thermoplastic shaping step, which two heating steps tend to produce more heat degradation of the matrix material.
Another object of the present invention, therefore, is to provide a method of producing shaped microporous articles having advantages in the above respects.